An RC circuit may include, for example, a circuit with a power or a voltage source (e.g., battery) connected to a resistor (R) and a capacitor (C). RC circuits are found, for example, in many different electronic circuits, e.g., filters and/or phase-locked loops, and may be included, for example, on microchips (“chips”) or circuit board-level components. A time constant of an RC circuit, i.e., the RC time constant, generally refers to a time needed for a voltage across the resistor/capacitor to rise (with respect to the capacitor) or fall (with respect to the resistor) to a defined percentage of a final charging or discharging value of the capacitor. The RC time constant thus depends at least on the resistance and capacitance, and, more particularly, is generally directly related to a size of each of R and C.
Construction and use of many on-chip RC circuits, such as, for example, RC filters, may benefit from an accurate time constant, in order, for example, to define associated filter transfer functions independently of, e.g., process variations and temperature fluctuations. In other words, on-chip RC circuits may be constructed and operated with the expectation that a time constant of an RC circuit will equal R*C, as expected in the ideal case. In reality, however, actual values of R and/or C within a given circuit may not match expected values, and, moreover, may change over a period of time (e.g., again, due to temperature fluctuations experienced by the circuit(s)).
Accordingly, RC circuits and related circuits may be calibrated, so that the RC circuit behaves in an expected manner in a known amount of time. For example, a variable resistance and/or variable capacitance may be used, so that periodic adjustments may be made to the RC circuit to cause the actual RC circuit components to function in a predictable way in an expected amount of time.
In one such technique for calibrating an RC circuit, a current mirror may first be used to make sure that the same current flows through a resistor and capacitor that are otherwise connected in parallel within the RC circuit. Then, the capacitance, which may be variable, may be adjusted until the voltages across the resistor and the capacitor equal one another after a time period (i.e., time constant) of R*C from an initial state of charge/discharge of the capacitor, as may be shown to be expected for such a configuration.
Such a current mirror, however, may create a large parasitic capacitance to ground. One technique for minimizing an effect of such a parasitic capacitance is to use a large capacitance for the capacitor of the RC circuit (thereby, relatively speaking, minimizing an effect of the parasitic capacitance). In order to have such a large capacitance, however, it may be necessary to dedicate a relatively large area of a chip on which the filter is constructed to the capacitor(s) in the RC circuit. In such cases, compensation for the parasitic capacitance may come at a cost of valuable and limited chip area.